I've only had one divert (owing to fog at departure airport), & the non-loco airline laid on a coach for us & drove us from Antwerp to Rotterdam, in time to get another of their later flights, & we arrived a couple of hours late at our original destination of LCY. Couldn't fault the service (and the sight of the red port-lights on the huge concrete towers at Rotterdam port just peeping above the nighttime fog, like giant alien eyes was something else).
However, do the likes of Ryanair, Easyjet etc. do this, or is it "Tough. Get yourself to another airport & we'll sell you/give you another flight". Not trying to bash anyone, I just don't know what the situation is.